Walking function has a critical role in life functions and health. According to the Americans with Disability: 2010 report from the US Census Bureau, roughly 30.6 million individuals aged 15 years and older had limitations associated with ambulation including difficulty walking. These limitations represent a significant healthcare, societal and economic problem, as these people are at risk of developing co-morbidities, rapidly declining health, and face significant challenges associated with integrating into the community and rejoining the workforce. Impaired ankle function is thought to be a major contributing factor to the reduced gait function in elderly and stoke survivors. Recent results suggest that ankle-assisting exosuits can improve gait after stroke. The Biodesign Lab has developed new soft exoskeleton systems (?exosuits?) that are constructed from compliant materials such as fabrics and transmit force from small actuator packs to ankle and hip joints. By triggering actuation to assist the user at carefully-selected phases of the gait cycle, these systems demonstrably reduce the energetic cost of locomotion and can help correct pathological gait. Compared to traditional rigid exoskeletons, these systems are lightweight, comfortable, and do not hinder normal joint motions. Numerous studies have confirmed that wearable ankle exoskeletons or exosuits can significantly lower the metabolic cost of locomotion and promote more effective gait. However, individual benefit varies widely and the assistance parameters (e.g. applied joint torque, timing) that work well for some individuals are counterproductive for others. State-of-the-art techniques such as empirical optimization are successful in finding the metabolic optimum but are time intensive and often limited to the gait condition tested. Additionally, these methods so far fail to provide a mechanistic explanation for differences in individual response which could be used to improve exosuit design and function. This proposal targets a new approach for quickly individualizing assistance through the development of low- profile and portable ultrasound imaging technology that can visualize and measure the behavior of muscles and tendons within the leg. The hypothesis is that direct measurement of the dynamics of the plantarflexor muscle- tendon unit (MTU) using ultrasound imaging will provide essential insight into the mechanisms that underlie human interaction with exosuit assistance. Furthermore, signals derived from MTU dynamics can enable effective individualized and adaptive exosuit assistance in diverse gait conditions. This developmental R21 project will result in the creation and validation of a system to measure the state of important MTU parameters in the leg during exosuit operation. The rich biomechanics dataset will provide insights into user response and will be made available to researchers. The preliminary work with elderly individuals will provide the framework for extending the potential of exosuit technology to a broader range of clinical users where assistive strategies are customized to the user and the demands of real-world locomotion.